The measurement of electrical potentials on the skin has many uses. For example, electrocardiograms are derived from measuring the potential generated by the heart of a patient at various points on the skin.
Skin potentials are also measured in apparatus for determining the electrical impedance of human tissue, including two-dimensional (e.g., U.S. Pat. Nos. 5,063,937, 4,291,708 and 4,458,694) or three-dimensional (e.g., U.S. Pat. Nos. 4,617,939 and 4,539,640) mapping of the tissue impedance of the body. In such systems an electrical potential is introduced at a point or points on the body and measured at other points at the body. Based on these measurements and on algorithms which have been developed over the past several decades, an impedance map or other indication of variations in impedance can be generated.
U.S. Pat. Nos. 4,291,708 and 4,458,694 and "Breast Cancer screening by impedance measurements" by G. Piperno et al. Frontiers Med. Biol. Eng., Vol. 2, pp 111-117, the disclosures of which are incorporated herein by reference, describe systems in which the impedance between a point on the surface of the skin and some reference point on the body of a patient is determined. These references describe the use of a multi-element probe for the detection of cancer, especially breast cancer, utilizing detected variations of impedance in the breast.
In these references a multi-element probe is described in which a series of flat, stainless steel, sensing elements are mounted onto a PVC base. A lead wire is connected between each of these elements and detector circuitry. Based on the impedance measured between the elements and a remote part of the body, signal processing circuitry determines the impedance variations in the breast. Based on the impedance determination, tumors, and especially malignant tumors, can be detected.
The multi-element probe is a critical component in this system and in other systems which use such probes. On one hand the individual elements must make good contact with the skin and with the corresponding points on the sensing or processing electronics while also being well isolated from each other. On the other hand, use of gels to improve skin contact carries the risk of cross-talk, dried gel build-up on the elements and inter-patient hygienic concerns.
A paper titled "Capacitive Sensors for In-Vivo Measurements of the Dielectric Properties of Biological materials" by Karunayake P. A. P. Esselle and Stanislaw S. Stuchly (IEEE Trans. Inst & Meas. Vol. 37, No. 1, p. 101-105) describes a single element probe for the measurement of in vivo and in vitro measurements of the dielectric properties of biological substances at radio and microwave frequencies. The sensor which is described is not suitable for impedance imaging.
A paper entitled "Messung der elektrischen Impedance von Organen-Apparative Ausrustung fur Forschung und klinishe Anwendung" by E. Gersing (Biomed. Technik 36 (1991), 6-11) describes a system which uses single element impedance probes for the measurement of the impedance of an organ. The device described is not suitable for impedance imaging.
A Paper titled "MESURE DE L'IMPEDANCE DES TISSUS HEPATIQUELES TRANSFORMES PAS DES PROCESSUS LESIONELS" by J. Vrana et al. (Ann. Gastroentreol. Hepetol., 1992, 28, no. 4, 165-168) describes a probe for assessing deep tissue by use of a thin injection electrode. The electrode was positioned by ultrasound and specimens were taken for cytological and histological assessment. The electrode was constituted on a biopsy needle used to take the samples.
A paper titled "Continuous impedance monitoring during CT-guided stereotactic surgery: relative value in cystic and solid lesions" by V. Rajshekhar (British Journal of Neurosurgery (1992) 6, 439-444) describes using an impedance probe having a single electrode to measure the impedance characteristics of lesions. The objective of the study was to use the measurements made in the lesions to determine the extent of the lesions and to localize the lesions more accurately. The probe is guided to the tumor by CT and four measurements were made within the lesion as the probe passed through the lesion. A biopsy of the lesion was performed using the outer sheath of the probe as a guide to position, after the probe itself was withdrawn.
A paper titled "Rigid and Flexible Thin-Film Multi-electrode Arrays for Transmural Cardiac Recording" by J. J. Mastrototaro et al. (IEEE TRANS. BIOMED. ENG. Vol. 39, No. 3, Mar. 1992, 271-279) describes a needle probe and a flat probe each having a plurality of electrodes for the measurement of electrical signals generated in the heart.
A paper entitled "Image-Based Display of Activation Patterns Derived from Scattered Electrodes" by D. S. Buckles et al. (IEEE TRANS. BIOMED ENGR. Vol. 42, No. 1, Jan. 1995, 111-115) describes a system for measurement of electrical signals generated on the heart by use of an array of electrodes on a substrate. The heart with the electrodes in place is viewed by a TV camera and an operator marks the positions of the electrodes on a display. The system then displays the heart (as visualized prior to the placement of the electrodes) with the position markings.
A paper entitled "Development of a Multiple Thin-Film Semimicro DC-Probe for Intracerebral Recordings" by G. A. Urban et al. (IEEE TRANS. BIOMED ENGR. Vol. 37, No. 10, Oct. 1990, 913-917) describes an elongate alumina ceramic probe having a series of electrodes along its length and circumference for measuring functional parameters (electrical signals) in the brain. Electrophysiological recording, together with electrostimulation at the target point during stereotactic surgery, was performed in order to ensure exact positioning of the probe after stereotactic calculation of the target point. Bidimensional X-Ray imaging was used in order to verify the exact positioning of the electrode tip.